This invention relates to data amplifiers having a bandwidth which is adaptively responsive to the data rate of the signal, and more particularly to a transimpedance preamplifier for a photodiode in an optical data communications system.
Optical communications systems are increasingly used for data communications because of their advantages, including low attenuation of the optical signal in a fiber optic cable, and because of the very wide bandwidth achievable, which allows a very high data rate. The optical or light signal propagating along a fiber optic cable may be periodically regenerated by a repeater. When the light signal arrives at the utilization station or at a regeneration station, it is converted into an electrical signal by a photodetector or photodiode. The photodetector generates an electrical signal current in response to the light signal. This current is often very small and may require preamplification before further operations related to regeneration or to generation of data may be performed.
As described in the article "PIN-FET Receiver for Fiber Optics" by Siegel et al., published March 1984 in the RCA Review at pp. 4-23, there are three major approaches to front-end design for an optical receiver. These include termination of the photodetector in a simple low resistance load resistor, termination in a high or integrating impedance, or termination in a transimpedance amplifier (a voltage amplifier having degenerative negative feedback). The simple low resistance input load is advantageous because it provides relatively broad bandwidth, even in the presence of substantial values of input capacitance. However, the low resistance results in large RMS noise currents, which adversely affects performance at low signal levels or in a long optical fiber communications system in which the light signal may be repeatedly converted to an electrical signal at each of a large number of regeneration stations.
It is desirable to keep the input resistance value high to minimize the contribution of thermal noise due to the resistance. The high integrating impedance technique is complex in that the design of the associated equalizer depends upon the signal spectrum and upon the coding format, and there are problems with dynamic range due to the large voltage which can occur.
The transimpedance or negative shunt feedback front end using a high-impedance input stage such as an FET is the most common approach to amplification of signals from photoelectric converters. It has the advantage of high dynamic range and good noise performance. The bandwidth of the photodetector-transimpedance amplifier depends on the total input capacitance at the amplifier input according to the equation ##EQU1## where R.sub.F is the value of the shunt negative feedback resistance, A is the closed loop gain of the voltage amplifier which forms the forward gain path of the transimpedance amplifier, C.sub.i is the total capacitance at the input junction of the photodetector and the transimpedance amplifier, and R.sub.L is the total impedance at the input junction, including the photodiode resistance, the input resistance of the input stage of the voltage amplifier, and the feedback resistor R.sub.F. Ordinarily, the photodiode is a high-impedance current source having a high resistance, and the input impedance of the voltage amplifier is also high, so that R.sub.L =R.sub.F. Under these conditions, the 3-dB cutoff frequency is EQU .omega..sub.3dB =A/R.sub.F C.sub.i
Circuit optimization consists of minimizing input capacitance C.sub.i, and then choosing the largest value of R.sub.F which meets the bandwidth requirements in view of C.sub.i. For a given value of C.sub.i established by the parallel capacitances of the photodetector and the input stage of the voltage amplifier, the data rate which must be handled establishes the bandwidth and therefore establishes the maximum allowable value of feedback resistance R.sub.F. This, in turn, establishes the noise floor of the receiver. According to the aforementioned article, a buffer stage may be added to the output end of the feedback loop to prevent any parasitic output capacitance from being reflected into the receiver input.
U.S. Pat. No. 4,415,803 issued Nov. 15, 1983, to Muoi describes an optical detector driving a transimpedance amplifier. Enhanced dynamic range is provided by a variable impedance shunt disposed at the amplifier input for shunting signal current to ground. An automatic gain control circuit produces a control signal which varies in response to the amplitude of the output voltage. The control signal is applied to the variable impedance device to vary the impedance thereof. The control voltage is generated by a clamp cascaded with a peak-to-peak detector responsive to the data signal at the output of the amplifier.